1. Field of Invention
This invention relates to printed circuit boards for electronic devices. More particularly, the invention relates to a device and method for minimizing bridging across the leads of integrated circuits, connectors or other closely spaced through-hole-components soldered to the board with the leads parallel to a soldering wave during a wave soldering process
2. Description of the Prior Art
Through-Hole Components (THC) wave soldered to Printed Wiring Boards (PWBs) continue to be used in many circuit packs designs. There is a need to continue to find ways to further reduce wave soldering defects for increasingly dense circuit packs. Through-hole components, such as connectors or sockets widely used on input/output boards for mechanical strength and cost reasons are very difficult to wave solder without high incidence of solder crosses. Because of this difficulty, most design guides require the longitudinal axes of the components along the longest row of leads to be oriented parallel to with the board direction passing through the solder wave. However, this requirement often imposes a severe handicap on board designers in placing components and routing circuitry for many high density boards. Quite often designers have to use a board with a higher layer count, a larger board, or two boards instead of one. The inefficient use of board real estate not only increases the cost of the product but also reduces their electrical and thermal performance. Moreover, some edge connectors can only be placed with their longest lead rows parallel to with the soldering wave or perpendicularly or in a "misoriented" direction to the board travel direction, violating required design rules. These "Disoriented" parts inevitably cause high incidence of solder crosses that need to be repaired. Sometimes they impede cooling current flow and generally result in less efficient use of circuit board real estate. Thieving pads on printed wiring boards have been used to reduce the amount of solder crosses in a wave soldering process for many years. However, the emphasis of their use has largely been placed on wave soldering of surface mounted components. Earlier attempts at using this method on through-hole components were not very successful for production purposes. Accordingly, a need exists for an improved solder thieving pad on printed circuit boards for wave soldered through-hole components where the leads of the components are parallel to with the soldering wave or perpendicular to the board direction passing through the soldering wave during the soldering operation.
Prior art related to thieving pads includes the following:
U.S. Pat. No. 5,736,680 entitled "Polymorphic Rectilinear Thieving Pad" to B. E. Caldwell et al., issued Apr. 7, 1998 (Caldwell) discloses a pad adjacent to a through-hole that will be located on the trailing edge of the board when the board is processed through wave soldering. The pad includes a curved side that faces the hole. In addition, corners are absent from the pad. The intersection of sides of the pad occurs as a curve or radiuses corner rather than a vertical corner. Wave soldering the printed circuit board with the pad reduces environmental conditions that induce bridging of a lead inserted into the hole adjacent the pad.
U.S. Pat. No. 5,679,929 entitled "Anti-Bridging Pads for Printed Circuit Boards and Interconnecting Substrates" to D. W. Greenfield et al., issued Oct. 21, 1997 (Greenifield) discloses anti-bridging pads for wave soldering of interconnecting substrates. The anti-bridging pads are disposed adjacent to trailing pads surrounding through holes and have geometries and/or placements with respect to the trailing pads in an array of functional pads which reduce or eliminate solder bridging.
U.S. Pat. No. 5,604,333 entitled "Process and Structure for a Solder Thief on Circuit Boards" to R. A. Keninish et al., issued Feb. 18, 1997 (Kennisli) discloses the formation of solder bridges between pads on circuit boards is minimized by a process and structure for wicking excess solder deposited during a wave soldering process. Solder thieves are placed adjacent to a last pad of a series of pads and are approximately the same width and twice the length as the pad in order to provide sufficient wicking of the solder. The excess solder is formed on the solder thieves and solder bridges are prevented.
U.S. Pat. No. 4,835,345 entitled "Printed Wiring Board Having Robber Pads for Excess Solder" to J. F. Haarde, issued May 30, 1989 (Haarde) discloses a printed wiring board which has one or more solder thieving pads of special configuration which receive and contain excess solder which might otherwise bridge adjacent downstream leads of a component having closely spaced leads when the component is mounted to the printed circuit board in a wave soldering process. The special solder thieving pad configuration may be viewed as an extra solder thieving pad (a "robber pad") at the downstream end of a linear array of solder thieving pads which is connected to the adjacent upstream solder thieving pad by a solder-wettable bridge.
U.S. Pat. No. 5,227,589 entitled "Plated-Through Interconnect Solder Thief" to A. R. Weeks, issued Jul. 13, 1993 (Weeks) discloses a plated-through hole solder thief for inhibiting solder bridges between adjacent leads of an integrated circuit and simultaneously providing for electrical interconnections of certain leads of the integrated circuit to other surfaces or layers of a printed wiring board. Plated through-holes are used as solder thieves and are located in the proximity of the last solder thieving pad in a row of solder thieving pads. The plated through-hole draws the solder the last solder thieving pad to fill the plated through-hole and inhibit bridging between functional leads of the integrated circuit as well as make an electrical interconnection between the last lead of the integrated circuit in each row and other surfaces or layers of the printed wiring board. The plated-through hole is located askew from the rows of solder thieving pads so that functional use may be made of the plated-through hole once it is filled to provide a test point for testing the integrated circuit.
U.S. Pat. No. 5,414,223 entitled "Solder thieving pad for Printed Circuit Boards" to E. D. Suski et al., issued May 9, 1995 (Suski) discloses an omni-directional non-occluding solder thieving pad design for printed circuit boards comprising a plurality of spokes radiating outward from a through-hole on a printed circuit board, with a ring concentric to the through-hole that intersects each of the plurality of spokes at approximately a perpendicular angle. The ring-spoke configuration eliminates tile need to insure proper orientation of tile solder thieving pad on the printed circuit board prior to a wave soldering process since the symmetrical ring-spoke design is omni-directional. The concentric ring structure provides an additional contact area of solder between the printed circuit board and the computer chassis. This additional contact of solder ensures that there is sufficient electrical connection between the printed circuit board and tile computer chassis such that when the printed circuit board is mounted to tile computer chassis, a proper grounding connection is provided.
None of the prior art discloses a multi leaded THC in a copper surfaced printed circuit board having solder thieving pads of a single simple geometry for all types of component leads and configuration in all types of board surface finishes which reduce "bridging" while tile thieving pads are parallel to a soldering wave or perpendicular or in a "misoriented" direction to the board travel direction in a wave soldering process, the pads taking into account the total molten solder volume held in tile solder joint; balancing the weight of the solder volume, the dynamic solder flow in the wave and the solder surface tension to improve the drainage behavior of the solder to inhibit solder bridging of the component leads.